Method and circuit for suppressing data loading noise in non-volatile memories

Abstract: In a nonvolatile memory (100) comprising a data amplifying unit (106) and an output element (108) mutually connected by a connection line (107), the noise suppressing circuit (1) comprises a network (8, 18) for generating a noise suppressing signal (N) which is synchronized perfectly with a signal (L) controlling data loading from the amplifying unit (106) to the output unit (108), presents a very short duration, equal to the switching time of the output unit (108), and freezes the amplifying unit during switching of the output unit to prevent this from altering the data stored in the amplifying unit or internal circuits of the memory (100). The same signal also blocks an address amplifying unit (102) on the address bus (101, 103).

Description: The present invention relates to a method and circuit for suppressing the data loading noise in nonvolatile memories. As is known, increasing demand exists for memories characterized by flexibility, low consumption and immunity to noise, which characteristics are extremely difficult to achieve simultaneously in that each can only be obtained at the expense of the others. As such, a trade-off is inevitable, bearing in mind the effect in terms of technological yield, and the impossibility of achieving an all-round solution suitable for all applications. A timer, for example, is useful in reducing consumption but, being invariably rigid, responds poorly to certain inevitable technological changes which are better catered to by a static type architecture. Moreover, a timed architecture, though it provides for solving certain noise, dissipation and speed problems, is limited in situations in which certain lines are brought only slowly up to the steady-state condition. Bearing in mind these and many other factors (e.g. slow locations, component characteristic shift, localized nonuniform behaviour), a timer has been proposed for regulating the duration of the load pulse according to the characteristics of the memory, and as described in co-pending patent application entitled "Load signal generating method and circuit for nonvolatile memories." This architecture provides for exploiting the advantages of the timed approach (low consumption, speed) while at the same time recovering slower devices, or devices with nontypical parameters or involving frequent resetting of lines which are slow to reach the correct steady-state level. On the other hand, a good degree of immunity to noise when loading data into the output circuits requires the generation of load pulses of as short a duration as possible, e.g. by normally maintaining the output circuits disconnected from the internal memory circuits, and only permitting a very brief connection when actually loading the data (as described in co-pending patent application entitled "Timing method and circuit for loading nonvolatile memory output data"). Noise immunity as described above, however, depends entirely on the load pulse being very short, and is lost or at any rate considerably reduced in the event the duration of the load pulses is extended for recovering slower devices and situations. It is an object of the present invention to provide a timer which, on the one hand, provides for achieving a good degree of immunity to noise, and, on the other, also permits the use of devices which are slower intrinsically or at least in certain operating conditions. According to the present invention, there is provided a method and circuit for suppressing the data loading noise in nonvolatile memories, as claimed respectively in Claims 1 and 5. In practice, according to the invention, the duration of the load pulse is modulated as required by the memory, and the outputs of the output and address buffers are frozen for a brief instant by means of a pulse enabled in perfect time with the load pulse, so that most of the output circuits and addresses switch without altering the data stored in the buffers, and the data relative to slower locations or situations (which, involving switching of far fewer elements, is not such as to generate noticeable noise) may then also be transmitted and acquired. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows one embodiment of the timing circuit according to the present invention; Figure 2 shows a plot of various signals in the Figure 1 circuit; Figure 3 shows a plot of various signals versus data switching. Number 1 in Figure 1 indicates a timing circuit forming part of a nonvolatile, conveniently flash type, memory 100 shown schematically and of which only a number of parts, timed by circuit 1, are shown. More specifically, of memory 100, Figure 1 shows: an external address bus 101; an address amplifying unit 102; an internal address bus 103; a memory cell array 104; an amplifying unit 105; a data amplifying unit 106; a data bus 107; and a latch type output unit 108. Address amplifying unit 102 is formed by a number of buffers, and is input-connected to external address bus 101, and output-connected to internal address bus 103. Unit 102 is feedback controlled by an address feedback bus 109 formed by a number of lines along which are provided switches controlled by a single control signal and of which only one is shown in the form of an N-channel transistor 110 with its gate terminal connected to a line 111 supplying signal N. Feedback by line 109 prevents the data stored in the buffers of unit 102 from switching in the event of noise on output bus 103 or switching on bus 101, by "freezing" it to the previous value. Internal address bus 103 is connected to an address decoding section 112. Array 104 and amplifying unit 105 are connected by a bus 115; unit 105 (formed by a number of sense amplifiers) and data amplifying unit 106 are connected by a bus 116; amplifying unit 106 (formed by a number of buffers) is feedback controlled by a data feedback bus 117 in turn formed by a number of lines along which are provided switches controlled by a single control signal and only one of which, 118, is shown with its control input connected to line 111. As in the case of line 109, feedback line 117 provides for blocking the data stored in the output buffers of unit 106 as long as switches 118 are closed, and regardless of any noise generated on data bus 116 by switching of output unit 108, by virtue of the buffer outputs of unit 106 presenting a much greater impedance with respect to the sense amplifier outputs of unit 105. Data bus 107, also formed by a number of lines, presents a number of switches controlled by a single signal and of which only one, 120, is shown. The control terminal of switch 120 is connected to a line 122 supplying load signal L, so as to normally interrupt data bus 107 except for the time necessary for loading the data. Output unit 108 is formed by a number of output circuits, each presenting two inputs 123, 124; each input 123 is connected to a respective line of data bus 107; and inputs 124 are all connected to an enabling line supplying a common enabling signal OE. For each output circuit, output unit 108 also presents two outputs 125, 126 connected to respective gate terminals of two transistors 127, 128. P-channel transistor 127 presents the source terminal connected to the supply (V DD ), and the drain terminal connected to an output node 129 to which is also connected the drain terminal of N-channel transistor 128, the source terminal of which is grounded. Circuit 1 comprises a single-shot (monostable) circuit 2 having an input 3 connected to line 122. Single-shot circuit 2, which is enabled by the trailing edge of signal L, generates at the output a pulse signal END supplied to the reset input R of a synchronizing and asymmetrical delay block 4 formed substantially by an asymmetrical delay flip-flop. Block 4 also presents a second set input S supplied with a synchronizing signal SYNC generated by other sections of memory 100 and which enables the data read phase. Block 4 responds rapidly to each reset request, and with a predetermined delay to each enabling request, for which purpose, on receiving the leading edge of the SYNC pulse, it switches immediately to the reset state, and, on receiving the trailing edge of the same pulse, controls a chain of delay elements. At the end of the delay determined by the delay element chain, the output of block 4 switches to high, and is reset to low on receiving signal R or a second SYNC pulse prior to receiving reset signal R. The output of block 4 (supplying data simulating signal SP) is connected to a node 5 and, via a controlled switch 6, to one input 7 of an out-like circuit 8 having a second input 9 connected to node 5 via an inverter 11, the output of which defines a node 10 supplying signal SS. Out-like circuit 9 presents the same structure as the output circuits constituting unit 108, so as to reproduce exactly the same propagation delay, and, like the output circuits of unit 108, also presents two outputs connected to the inputs of a NAND gate 15 and to respective transistors 16, 17. P-channel transistor 16 presents short-circuited drain and source terminals connected to supply line V DD , while transistor 17 presents short-circuited, grounded drain and source terminals, both having the same geometry as, and simulate the capacitance of, transistors 127, 128. The output of NAND gate 15 (supplying signal EP) is connected to one input of an AND gate 18 and to one input 19 of a pulse duration modulating block 20. AND circuit 18 presents a second input connected to node 5; and an output connected to line 111 to which it supplies signal N. Block 20 presents a second input 21 connected to node 10; and inputs 22-25 supplied with signals SB, V IN' SC and CNT. As described in detail in the above cited co-pending patent entitled "Load signal generating method and circuit for nonvolatile memories", block 20, on the basis of the logic value of signals SB, SC, CNT and the value of voltage V IN related to the supply voltage, generates signal L at output 29 connected to line 122, which signal L presents a leading edge synchronized perfectly with that of signal N, and a duration depending on the condition of the circuit. More specifically, if extension of the load pulse is disabled (CNT low), if voltage V IN is at the rated value, if the circuit is not in low-power operating mode (standby state, signal SB low), and if static operating mode is not requested (SC low), the trailing edge of signal L is simultaneous with signal N. Conversely, if extension of the load pulse is enabled (CNT high), or at the end of a critical operating condition, the trailing edge of signal L is delayed with respect to N, as described below with reference to Figure 2. If, on the other hand, static operating mode is requested (SC high), or the circuit is in standby mode (SB high), or voltage V IN is below a predetermined threshold, signal L remains high as long as the system remains in the above state, but switches back to low with a delay with respect to switching of respective signal SC, SB or return of V IN to the acceptable threshold level. Operation of the circuit will now be described with reference to Figure 2. Supposing initially that extension of the pulse duration is disabled (CNT low). When idle, signals SYNC, SP, N, L, END are low, and signals SS, EP high. At instant t 0 , memory 100 supplies a SYNC pulse, and with a delay determined by block 4 (and depending on the precharging and evaluation times of array 100), signal SP switches (instant t 1 ), so that AND circuit 18 switches, the output signal N of which switches to high, signal SS switches to low, load signal L switches to high, and switches 6, 110, 118 and 120 are closed. Closure of switch 6 provides for supplying data simulating signal SP to input 7 of out-like circuit 8; and closure of switch 118 for freezing and so preventing undesired switching of the output buffers of unit 106. At the same time, transmission commences of the data frozen in the output buffers of unit 106 to the output circuits (unit 108) which thus switch. Since, as already stated, output buffers 106 are prevented from switching at this phase, the noise generated by such switching (which involves high currents due to charging and discharging of the capacitive elements of the output circuits, and may thus also alter the voltage levels in amplifiers 105) is prevented from damaging the data. Similarly, the addresses amplified in unit 102 are also frozen to prevent switching of the addressed memory locations during reading. Signal SP thus propagates in out-like circuit 8 in the same way and simultaneously with the data in output circuits 108, and, at the end of propagation, the outputs of out-like circuit 8 switch to high, thus switching NAND circuit 15 (instant t 2 ); signal EP switches to low, thus switching AND circuit 18; and signal N switches back to low, thus opening switches 6, 110 and 118. Switching of signal EP is detected by block 20 which, as the pulse duration extension function is disabled, switches load signal L to low, thus also opening switch 120 and terminating the data loading phase, which therefore lasts precisely as long as necessary for propagation of the data in output circuit 108. Output circuits 108 then remain in the just memorized condition, with no change in the stored data, and are unaffected by any noise. On receiving the trailing edge of signal L, single-shot circuit 2 generates at the output a pulse (END signal) for resetting block 4. With a given delay, at instant t 3 , signal SP switches back to low, thus switching signal SS to high and resetting out-like circuit 8 which, after a given delay, switches its outputs to low, thus switching circuit 15, so that signal EP switches to high to restore the initial conditions (instant t 4 ). If, on the other hand, a SYNC signal pulse is received before the initial state is restored (instant t 4 ), block 4 switches immediately to the reset state and starts a further read phase, thus ensuring that each SYNC signal pulse starts a read cycle with a constant delay. Operation as described above is repeated whenever a sync pulse is received, and as long as signal CNT is low, and no static, standby or low-power operating conditions are present. If, on the other hand, extension of the load signal is enabled, data loading into output circuit unit 108 continues even after the end of the signal N pulse, as described below. Assuming signal CNT switches to high at instant t 5 , in this case, as previously, on receiving the sync pulse (instant t 6 ), signal SP switches to high with a given delay (instant t 7 ); SS switches to low; signals N and L switch to high; switches 6, 110, 118 and 120 are closed; transmission commences of the data simulating signal in out-like circuit 8, and the data in output unit 108; and, at the same time, the data in address buffers 102 and output buffers 106 is frozen to render it immune to noise. As previously, when the outputs of circuit 8 and NAND gate 15 switch, signal EP switches to low (instant t 8 ), the signal N pulse is terminated, and switches 6, 110 and 118 open. In this case, however, load signal L remains high and switch 120 closed, so that any delayed data supplied by sense amplifiers switching after most of the others may be loaded by the output buffers of unit 106 into corresponding output circuits of unit 108, as explained with reference also to Figure 3 which shows plots of the data on bus 107 (D 107 ) and at the outputs of output unit 108 (D OUT ). As shown by the continuous line, most of the data D 107 on bus 107 switches at the end of the evaluation phase by sense amplifiers 105, which switching will be assumed to be generally terminated at instant t 12 . At instant t 7 , determined by block 4 on the basis of standard evaluation times, the signal N and L pulses commence simultaneously with data loading into output unit 108, the outputs of which begin switching (data D OUT ). As already stated, switching of a large number of circuits involves high currents which could possibly damage the data stored in buffers 106, but which is prevented from doing so by freezing the data in buffers 106. Pulse N is terminated upon complete switching of the outputs of output circuit 108 (corresponding to complete switching of the outputs of out-like circuit 8). Should further amplifiers 105 switch in the meantime or successively, this may now be propagated to the output buffers as shown by the dotted line at instant t 13 in Figure 3. In view of the small amount of current involved, no damage is caused by subsequent switching of the respective output circuits (instant t 14 ). At instant t 9 , signal L also switches to low, thus opening switches 120 and separating the output circuits from buffer unit 106; and, as in the previous case described above, the trailing edge of signal L enables single-shot circuit 2 which generates an END signal pulse to reset block 4. After a given time (instant t 10 ), SP and SS switch to also reset out-like circuit 8; and, at instant t 11 , signal EP also switches to high to restore circuit 1 to the idle condition. The advantages of the circuit described are as follows. Firstly, it provides for timing which is both flexible and immune to noise, acting on the most significant points which are brought to a block condition when present data may be damaged by switching of the output circuits, while at the same time maintaining the loading phase as long as necessary for also loading the data of the slower elements. Moreover, connection of output unit 108 and amplifying unit 106 is interrupted at the end of the loading phase, to prevent any noise on the lines affecting the data. The signal for freezing the data (and address) buffers is synchronized perfectly with the load signal to ensure the data is frozen at the exact time it is loaded. Moreover, the load signal (which, via synchronizing circuit 4, is only enabled when the data of most of the sense amplifiers of unit 105 has been read) presents a duration which, the characteristics of the memory permitting, is the minimum for ensuring switching of the outputs, thanks to out-like circuit 8, and is only extended when necessary (e.g. on exiting critical situations, in which case the memory undoubtedly presents delays in reaching steady-state values, or in the presence of slow elements). Clearly, changes may be made to the circuit and method as described and illustrated herein without, however, departing from the scope of the present invention.

Claims: A method of suppressing the data loading noise of nonvolatile memories, comprising the step of enabling a data loading signal (L) for loading data from a data amplifying unit (106) to an output element (108); characterized in that it comprises the steps of: enabling a noise suppressing signal (N) simultaneously with said step of generating said data loading signal (L); and blocking switching of said data amplifying unit (106) for a suppression time substantially equal to the switching delay of said output element (108), said blocking being obtained by feeding back the output of the amplifying unit to its input under control of said noise suppressing signal (N). A method as claimed in Claim 1, characterized in that it comprises the step of enabling address loading from an address amplifying unit (102) to an address processing unit (112), and simultaneously blocking switching of said address amplifying unit for said suppression time. A method as claimed in Claim 1 or 2, characterized in that it comprises the step of disabling said noise suppressing signal, for deblocking switching of said data amplifying unit (106) at the end of said suppression time, and maintaining said loading signal enabled. A method as claimed in Claim 3, characterized in that it comprises the steps of generating a data simulating signal (SP); enabling loading of said data simulating signal into an out-like circuit (8), identical to said output element (108), simultaneously with said step of enabling data loading into said output element; and deblocking switching of said data amplifying unit (106) following propagation of said data simulating signal in said out-like circuit. A data loading noise suppressing circuit in a nonvolatile memory (100) comprising a data amplifying unit (106) and an output element (108) mutually connected by a connection line (107), said data loading noise suppressing circuit (1) comprising enabling means (4) for generating an enabling signal (SP); loading generator means (20) having an enabling input connected to said enabling means, and generating a loading signal (L); and output enabling means (120) for enabling data loading from said data amplifying unit (106) to said output element (108) on detecting said loading signal; characterized in that it comprises noise suppressing means (8, 18, 117, 118) including suppressing signal generator means (18) having an enabling input connected to said enabling means (4) and generating a noise suppressing signal (N) enabled simultaneously with said loading signal (L); data blocking means (117, 118) controlled by said noise suppressing signal (N) and connected between the output and the input of said data amplifying unit (106) for preventing switching of said data amplifying unit in the presence of said noise suppressing signal (N); and disabling means (8) for generating a disabling signal (EP) for said suppressing signal generator means (18) at the end of the switching delay of said output element (108). A circuit as claimed in Claim 5, in a nonvolatile memory (100) also comprising an address amplifying unit (102) and an address processing unit (112) connected by an address line (103); characterized in that it also comprises address blocking means (109, 110) connected to said address amplifying unit (102), for preventing switching of said address amplifying unit in the presence of said noise suppressing signal (N). A circuit as claimed in Claim 5 or 6, characterized in that said enabling means comprise simulating signal generator means (4) for generating a data simulating signal (SP); and said disabling means comprise an out-like circuit (8) identical to said output element (108) and having an input (7) connected to said simulating signal generator means (4) via switching means (6) controlled by said noise suppressing signal (N). A circuit as claimed in Claim 7, characterized in that said loading generator means comprise loading extension means (20) having a first input (21) connected to said simulating signal generator means (4); a second input (19) connected to an output of said disabling means (8); a third input (25) supplied with an extension control signal (CNT); and an output (29) delivering said loading signal (L) to said output enabling means (120); said loading signal (L) being enabled simultaneously with said noise suppressing signal (N), and presenting the same duration as said noise suppressing signal in the absence of said extension control signal (CNT), and a longer duration in the presence of said extension control signal. A circuit as claimed in Claim 8, characterized in that said output (29) of said loading extension means (20) is connected to the reset input of said simulating signal generator means (4), for resetting the control circuit (1) upon disabling of said loading signal (L). A circuit as claimed in Claim 8 or 9, characterized in that said output enabling means comprise a controlled switch (120) located along a data line (107) connecting said data amplifying unit (106) and said output element (108), and having a control input connected to said output (29) of said loading extension means (20). A circuit as claimed in any one of the foregoing Claims from 5 to 10, characterized in that said data blocking means comprise a feedback line (117) connecting the input and output of said data amplifying unit (106); and controlled switch means (118) located along said feedback line and having a control input supplied with said noise suppressing signal (N). A circuit as claimed in any one of the foregoing Claims from 7 to 11, characterized in that said simulating signal generator means (4) comprise an asymmetrical-delay memory element for generating said data simulating signal (SP) with a predetermined delay following reception of an external control signal (SYNC), and which switches rapidly to the reset condition.

Inventors: GOLLA CARLA MARIA, MACCARRONE MARCO, OLIVO MARCO, PASCUCCI LUIGI, GOLLA, CARLA MARIA, MACCARRONE, MARCO, OLIVO, MARCO, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G11C7
IPC codes: G11C17, G11C11, G11C16, G11C7